Following are publications which disclose background information related to the present invention. These publications are discussed in greater depth in the Background sections indicated. Restriction maps of Ri plasmids are disclosed by G. A. Huffman et el. (1984) J. Bacteriol. 157:269-276; L. Jouanin (1984) Plasmid 12:91-102; and M. Pomponi et el. (1983) Plasmid 10:119-129 (see TIP Plasmid DNA). L. Herrera-Estrella et al. (1983) Nature 303:209-213, provides examples of use of the nos promoter to drive expression in plants of heterologous foreign structural genes. N. Murai et el. (1983) Science 222:476-482, reported the ocs promoter could drive expression of an intron-containing fusion gene having foreign coding sequences. (Manipulations of the TIP Plasmids). R. F. Barker et el. (1983) Plant Molec. Biol. 2:335-350, and R. F. Barker and J. D. Kemp, U.S. patent application Ser. No. 553,786 disclose the complete sequence of the T-DNA from the octopine-type plasmid pTi15955; homologous published sequences of other Ti plasmid genes are referenced therein. Barker and Kemp also taught use of various octopine T-DNA promoters to drive expression in plants of various structural genes (Genes on the TIP Plasmids).
Shuttle Vectors
Shuttle vectors, developed by G. B. Ruvkun and F. M. Ausubel (1981) Nature 289:85-88, which provide means for inserting foreign genetic material into large DNA molecules, include copies of recipient genome DNA sequences into which the foreign genetic material is inserted. Shuttle vectors can be introduced a recipient cell, by well known methods, including the tri-parental mating technique (Ruvkin and Ausubel, supra), direct transfer of a self-mobilizable vector in a bi-parental mating, direct uptake of exogenous DNA by Agrobacterium cells ("transformation"), spheroplast fusion of Agrobacterium with another bacterial cell, uptake of liposome-encapuslated DNA. After a shuttle vector is introduced into a recipient cell, possible events include a double cross-over with one recombinational event on either side of the marker (homogenotization). Phenotypically dominant traits may be introduced by single cross-over events (cointegration) (A. Caplan et al. (1983) Science 2.22.:815-821; R. B. Horsch et al. (1984) Science 223:496-498); one must guard against deletion of the resulting tandem duplication. Shuttle vectors have proved useful in manipulation of Agrobacterium plasmids.
"Suicide Vectors" (e.g. R. Simon et al. (1983) Biotechnol. 1:784-791), are shuttle vectors having replicons not independently maintainable within the recipient cell. Use of suicide vectors to transfer DNA sequences into a Ti plasmid has been reported (e.g. E. Van Haute et al. (1983) EMBO J. 2:411-417; L. Comai et al. (1983) Plasmid 10:21-30; P. Zambryski et al, (1983) EMBO J. 2:2143-2150; P. Zambryski et al. (1984) in Genetic Engineering, Principles, and Methods, 6, eds: A. Hollaender and J. Setlow; P. Zahm et al. (1984) Mol. Gen. Genet. 194:188-194; and Caplan et al., supra; and C. H. Shaw et al. (1983) Gene 28:315-330.
Overview of Agrobacterium
Included within the gram-negative genus Agrobacterium are the species A. tumefaciens and A. rhizogenes, respectively the causal agents of crown gall disease and hairy root disease of gymnosperm and dicotyledonous angiosperm plants. In both diseases, the inappropriately growing plant tissue usually produces one or more amino acid derivatives known as opines which may be classified into families whose type members include octopine, nopaline, mannopine, and agropine.
Virulent strains of Agrobacterium harbor large plasmids known as Ti (tumor-inducing) plasmids (pTi) in A. tumefaciens and Ri (root-inducing) plasmids in A. rhizogenes (pRi), often classified by the opine which they caused to be synthesized. Ti and Ri plasmids both contain DNA sequences, referred to as T-DNA (transferred-DNA), which in tumors are found to be integrated into the genome of the host plant. Several T-DNA genes are under control of T-DNA promoters which resembles the canonical eukaryotic promoter in structure. The Ti plasmid also carries genes outside the T-DNA region. The set of genes and DNA sequences responsible for transforming the plant cell are hereinafter collectively referred to as the transformation-inducing principle (TIP). The term TIP therefore includes, but is not limited to, both Ti and Ri plasmids.
General reviews of Agrobacterium-caused disease include those by D. J. Merlo (1982), Adv. Plant Pathol. 1:139-178; L. W. Ream and M. P. Gordon (1982), Science 218:854-859; M. W. Bevan and M.-D. Chilton (1982), Ann. Rev. Genet. 16:357-384; G. Kahl and J. Schell (1982) Molecular Biology of Plant Tumors; K. A. Barton and M.-D. Chilton (1983) Meth. Enzymol. 101:527-539; A. Depicker et al. (1983) in Genetic Engineering of Plants: an Agricultural Perspective, eds: T. Kosuge et al., pp. 143-176; A. Caplan et al. (1983) Science 222:815-821; T. C. Hall et. al., European Patent application 126,546; and A. N. Binns (1984) Oxford Surveys Plant Mol. Cell Biol. 1:130-160. A number of more specialized reviews can be found in A. Puhler, ed. (1983) Molecular Genetics of the Bacteria-Plant Interaction, including a treatment by D. Tepfer of A. rhizogenes-mediated transformation (pp. 248-258). R. A. Schilperoort (1984) in Efficiency in Plant Breeding. (Proc. 10th Congr. Eur. Assoc. Res. Plant Breeding), eds: W. Lange et al., pp. 251-285, discusses the Agrobacterium-based plant transformation in the context of the art of plant genetic engineering and plant improvement.
Infection of Plant Tissues
Plant cells can be transformed by Agrobacterium by several methods known to the art. For a review of recent work, see K. Syono (1984) Oxford Surveys Plant Mol. Cell Biol. 1:217-219. In the present invention, any method will suffice as long as the gene is stably transmitted through mitosis and meiosis.
The infection of plant tissue by Agrobacterium is a simple technique well known to those skilled in the art. Typically after being wounded, a plant is inoculated with a suspension of tumor-inducing bacteria. Alternatively, tissue pieces are inoculated, e.g. leaf disks (R. B. Horsch et al. (1985) Science 227.:1229-1231) or inverted stem segments (K. A. Barton et al. (1983) Cell 32:1033-1043). After induction, the tumors can be placed in tissue culture on media lacking phytohormones usually included for culture of untransformed plant tissue. Traditional inoculation and culture techniques may be modified for use of disarmed T-DNA vectors incapable of inducing hormone independent growth (e.g. see P. Zambryski et al. (1984) in Genetic Engineering, Principles, and Methods, 6, eds.: A. Hollaender and J. Setlow).
Agrobacterium is also capable of infecting isolated cells, cells grown in culture, callus cells, and isolated protoplasts (e.g. R.B. Horsch and R. T. Fraley (1983) in Advances in Gene Technology: Molecular Genetics of Plants and Animals (Miami Winter Symposium 20), eds.: K. Downey et al., p. 576; R. T. Fraley et al. (1984) Plant Mol. Biol. 3:371-378; R. T. Fraley and R. B. Horsch (1983) in Genetic Engineering of Plants: an Agricultural Perspective, eds.: T. Kosuge et al., pp. 177-194; A. Muller et al. (1983) Biochem. Biophys. Res. Comm. 123:458-462). The transformation frequency of inoculated callus pieces can be increased by addition of an opine or opine precursors (L. M. Cello and W. L. Olsen, U.S. Pat. No. 4,459,355).
Plant protoplasts can be transformed by the direct uptake of TIP DNA in the presence of a polycation, polyethelene glycol, or both (e.g. F. A. Krens et al. (1982) Nature 296:72-74), though integrated Ti plasmid may include non-T-DNA sequences.
An alternative method involves uptake of DNA surrounded by membranes. pTi-DNA may be introduced via. liposomes or by fusion of plant and bacterial cells after removal of their respective cell walls (e.g. R. Hain et al. (1984) Plant Cell Rept. 3:60-64). Plant protoplasts can take up cell wall delimited Agrobacterium cells. T-DNA can be transmitted to tissue regenerated from fused protoplasts.
The host range of crown gall pathogenesis may be influenced by T-DNA-encoded functions such as onc genes (A. Hoekema et al. (1984) J. Bacteriol. 158:383-385; A. Hoekema et al. (1:984) EMBO J. 3:3043-3047; W. C. Buchholz and M. F. Thomasshow (1984) 160:327-332). R. L. Ausich, European Patent Application 108,580, reports transfer of T-DNA from A. tumefaciens to green algal cells, and expression therein of octopine synthase and Tn5 kanamycin resistance genes. G. M. S. Hooykaasvan Slogteren et al. (1984) Nature 311:763-764, and J.-P. Hernalsteens al. (1984) EMBO J. 3:3039-3041, have demonstrated transformation of monocot cells by Agrobacterium without the customary tumorigenesis.
Regeneration of Plants
Differentiated plant tissues with normal morphology have been obtained from crown gall tumors. For example, L. Otten et al. (1981) Molec Gen. Genet. 183:209-213, used tms (shoot-inducing, root-suppressing) Ti plasmid routants to create tumors which proliferated shoots that formed self-fertile flowers. The resultant seeds germinated into plants which contained T-DNA and made opines. The tms phenotype can be partly overcome by washing of the rooting area and can be bypassed by grafting onto a normal stock (A. Wostemeyer et al. (1984) Mol. Gen. Genet. 194:500-507). Similar experiments with a tmr (root-inducing, shoot-suppressing) mutant showed that full-length T-DNA could be transmitted through meiosis to progeny and that in those progeny nopaline genes could be expressed, though at variable levels (K. A. Barton et al. (1983) Cell 32:1033-1043).
Genes involved in opine anabolism were capable of passing through meiosis, though the plants were male sterile if the T-DNA was not disarmed. Seemingly unaltered T-DNA and functional foreign genes can be inherited in a dominant, closely linked, Mendelian fashion. Genetically, T-DNA genes are closely linked in regenerated plants (A. Wostemeyer et al. (1984) Mol. Gen. Genet. 194:500-507; R. B. Horsch et al. (1984) Science 223:496-498; D. Tepfer (1984) Cell 37:959-967).
The epigenetic state of the plant cells initially transformed can affect regeneration potential (G. M. S. van Slogteren et al. (1983) Plant Mol. Biol. 2:321-333).
Roots resulting from transformation from A. rhizogenes have proven relatively easy to regenerate directly into plantlets (M.-D. Chilton et al. (1982) Nature 295:432-434; D. Tepfer (1984) Cell 37:959-967; Tepfer (1983) in Puhler, supra), and are easily cloned. Regenerability from transformed roots may be dependent on T-DNA copy-number (C. David et al. (1984) Biotechnol. 2:73-76). Hairy root regenerants have a rhizogenic potential and isozyme pattern not found in untransformed plants (P. Costantino et al. (1984) J. Mol. Appl. Genet. 2:465-470). The phenotype of these plants is generally altered, although not necessarily deleteriously.
Genes on the TIP Plasmids
The complete sequence of the T-DNA of an octopine-type plasmid found in ATCC 15955, pTi15955, has been reported (R. F. Barker et al. (1983) Plant Molec. Biol. 2:335-350), as has that of the T.sub.L region of pTiAch5 (J. Gielen et al. (1984) EMBO J. 3:835-846). Published T-DNA genes do not contain introns and do have sequences that resemble canonical eukaryotic promoter elements and polyadenylation sites.
Ti plasmids having mutations in the genes tms, tmr, tml, and ocs respectively incite tumorous calli of Nicotiana tabacum which generate shoots, proliferate roots, are larger than normal, and do not synthesize octopine; all but ocs are onc (oncogenicity) genes. In other hosts, mutants of these genes can induce different phenotypes (see M. W. Bevan and M.-D. Chilton (1982) Ann. Rev. Genet. 16:357-384). Mutations in T-DNA genes do not seem to affect the insertion of T-DNA into the plant genome (J. Leemans et al. (1982) EMBO J. 1:147-152; L. W. Ream et al. (1983) Proc. Natl. Acad. Sci. USA 80:1660-1664).
Octopine Ti plasmids carry an ocs gene which encodes octopine synthase (lysopine dehydrogenase). All upstream signals necessary for expression of the ocs gene are found within 295 bp of the ocs transcriptional start site (C. Koncz et al. (1983) EMBO J. 2:1597-1603). P. Dhaese et al. (1983) EMBO J. 2:419-426, reported the utilization of various polyadenylation sites by "transcript 7" (ORF3 of Barker et al., supra) and ocs. The presence of the enzyme octopine synthase within a tissue can protect that tissue from the toxic effect of various amino acid analogs (G. A. Dahl and J. Tempe (1983) Theor. Appl. Genet. 66:233-239; M. G. Koziel et al. (1984) J. Mol. Appl. Genet. 2:549-562).
Nopaline Ti plasmids encode the nopaline synthase gene (nos) (sequenced by A. Depicker et al. (1982) J. Mol. Appl. Genet. 1:561-573). The "CAAT" box, but not upstream sequences therefrom, is required for wild-type levels of nos expression; a partial or complete "TATA" box supports very low level nos activity (C. H. Shaw et al. (1984) Nucl. Acids Res. 12:7831-7846). Genes equivalent to tms and tmr have been identified on a nopaline-type plasmid and a number of transcripts have been mapped (L. Willmitzer et al. (1983) Cell 32:1045-1056).
Transcription from hairy root T-DNA has also been detected (L. Willmitzer et al. (1982) Mol. Gen. Genet. 186:16-22). Ri plasmids and tms Ti plasmids can complement each other when inoculated onto plants, resulting in calli capable of hormone-independent growth (G. M. S. van Slogteren (1983) Ph.D. thesis, Rijksuniversiteit te Leiden, Netherlands).
TIP plasmid genes outside of the T-DNA region include the vir genes, which when mutated result in an avirulent Ti plasmid. Several vir genes have been accurately mapped and have been found to be located in regions conserved among various Ti plasmids (V. N. Iyer et al. (1982) Mol. Gen. Genet. 188:418-424). The vir genes function in trans, being capable of causing the transformation of plant cells with T-DNA of a different plasmid type and physically located on another plasmid (e.g. A. J. de Framond et al. (1983) Biotechnol. 1:262-269; A. Hoekema et al. (1983) Nature 303:179-180; J. Hille et al. (1984) J. Bacteriol. 158:754-756; A. Hoekema et al. (1984) J. Bacteriol. 158:383-385); such arrangements are known as binary systems. Chilton et al. (18 Jan. 1983) 15th Miami Winter Syrup., described a "micro-Ti" plasmid made by resectioning the "mini-Ti" of de Framond et al., supra (see European Patent application 126,546 for a description). G. A. Dahl et al., U.S. patent application Ser. No. 532,280, and A. Hoekema (1985) Ph.D. Thesis, Rijksuniversiteit te Leiden, The Netherlands, disclose micro-Ti plasmids carrying ocs genes constructed from pTi15955. M. Bevan (1984) Nucl. Acids Res. 12:8711-8721, discloses a kanamycin-resistant micro-Ti. T-DNA need not be on a plasmid to transform a plant cell; chromosomally located T-DNA is functional (A. Hoekema et al. (1984) EMBO J. 3:2485-2490). Ti plasmid-determined characteristics have been reviewed by Merlo, supra (see especially Table II therein), and Ream and Gordon, supra.
TIP Plasmid DNA
Ri plasmids have been shown to have extensive homology among themselves (P. Costantino et al. (1981) Plasmid 5:170-182), and to both octopine (F. F. White and E. W. Nester (1980) J. Bacteriol. 144:710-720) and nopaline (G. Risuleo et al. (1982) Plasmid 7:45-51) Ti plasmids, primarily in regions encoding vir genes, replication functions, and opine metabolism functions (L. Jouanin (1984) Plasmid 12:91-102; K. Lahners et al. (1984) Plasmid 11:130-140; E. E. Hood et al. (1984) Biotechnol. 2:702-709; F. Leach (1983) Ph.D. Thesis, Universite de Paris-Sud, Centre d'Orsay, France); none of the homologies are in pRi T.sub.L -DNA. pRi T-DNA contains extensive though weak homologies to T-DNA from both types of Ti plasmid (L. Willmitzer et al. (1982) Mol. Gen. Genet. 186:16-22). DNA from several plant species contains sequences, referred to as cT-DNA (cellular T-DNA), having homology with the Ri plasmid (F. F. White et al. (1983) Nature 301:348-350, L. Spano et al. (1982) Plant Molec. Biol. 1:291-300; D. Tepfer (1982) in 2e Colloque sur les Recherches Fruitieres Bordeaux, pp. 47-59). G. A. Huffre.an et al. (1984) J. Bacteriol. 157:269-276 and Jouanin, supra, and Leach, supra, have shown that, in the region of cross-hybridization, the Ri plasmid pRiA4.sub.b is more closely related to a pTiA6 (octopine-type) than pTiT37 (nopaline-type) and that this Ri plasmid appears to carry sequence homologous to tms but not tmr. Their results also suggested that Ri T-DNA may be discontinuous, analogous to the case with octopine T-DNA (see below). The restriction maps of pRiA4.sub.b, pRi1855, and pRiHRI were respectively disclosed by Huffman et al., supra, M. Pomponi et al. (1983) Plasmid 10:119-129, and L. Jouanin supra. Ri plasmids are often characterizable as being agropine-type or mannopine-type (A. Petit et al. (1983) Mol. Gen. Genet. 190:204-214).
A portion of the Ti or Ri plasmid is found in the DNA of tumorous plant cells. T-DNA may be integrated (i.e. inserted) into host DNA at multiple sites in the nucleus. Flanking plant DNA may be either repeated or low copy number sequences. Integrated T-DNA can be found in either direct or inverted tandem arrays and can be separated by spacers. Much non-T-DNA Ti plasmid DNA appears to be transferred into the plant cell prior to T-DNA integration (H. Joos et al. (1983) EMBO J. 2:2151-2160). T-DNA has direct repeats of about 25 base pairs associated with the borders, i.e. with the T-DNA/plant DNA junctions, which may be involved in either transfer from Agrobacterium or integration into the host genome.
Ri plasmids integrate two separate T-DNAs, T.sub.L -DNA and T.sub.R -DNA, left and right T-DNAs, respectively. T.sub.L (about 15-20 kbp) and T.sub.R (about 8-10 kbp) are separated by about 15-20 kbp (Huffman et al., supra, Jouanin, supra). The region of agropine-type pRi T.sub.L and T.sub.R integrated can vary between individual plants or species inoculated (F. F. White et al. (1983) Nature 301:348-350; D. A. Tepfer (1984) Cell 37:959-967). Though T-DNA is occasionally deleted after integration in the plant genome, it is generally stable. Tumors containing a mixture of cells which differ in T-DNA organization or copy number are the result of multiple transformation events.
The exact location relative to the border repeats of T-DNA/flanking plant DNA junctions varies and need not be within a border repeat. Virulence is not always eliminated after deletion of one of either of the usual nopaline T-DNA border sequences (compare H. Joos et al. (1983) Cell 32:1057-1067 with K. Wang et al. (1984) Cell 38:455-462 and C. H Shaw et al. (1984) Nucl. Acids Res. 12:6031-6041, concerning the right border). The orientation of the right nopaline-border can be reversed without total loss of functionality, and a single border sequence .is capable of transforming closely-linked sequences (M. De Block et al. (1984) EMBO J. 3:1681-1689). A synthetic 25 bp hopaline right border repeat is functional (Wang et al., supra). Circular intermediates associated with T-DNA transfer appear to be spliced precisely within the 25 bp direct repeats (Z. Koukolikova-Nicola et al. (1985) Nature 313:191-196).
Manipulations of the TIP Plasmids
Altered DNA sequences, including deletions, may be inserted into TIP plasmids (see Shuttle Vectors). Some pTi derivatives can be transferred to E. coli and mutagenized therein (J. Hille et al. (1983) J. Bacteriol. 154:693-701). P. Zambryski et al. (1983) EMBO J. 2:2143-2150, report use of a vector, deleted for most T-DNA genes to transform tobacco and regenerate morphologically normal plants.
The nopaline synthase promoter can drive expression of drug resistance structural genes useful for selection of transformed plant cells. M. W. Bevan et al. (1983) Nature 304:184-187; R. T. Fraley et al. (1983) Proc. Natl. Acad. Sci. USA 80:4803-4807; and L. Herrera-Estrella et al. (1983) EMBO J. 2:987-995, have inserted the bacterial kanamycin resistance structural gene (neomycin phosphotransferase II, NPT2), or kan, from Tn5 downstream from (i.e. behind or under control of) the hopaline synthase promoter. The constructions were used to transform plant cells which in culture were resistant to kanamycin and its analogs such as neomycin and G418. Promoters for octopine T.sub.L genes ORF24 and ORF25 can also drive kan structural gene expression (J. Velten et al. (1984) EMBO J. 3:2723-2730). Herrera-Estrella et al., supra, reported a similar construction, in which a methotrexate resistance gene (dihydrofolate reductase, DHFR) from Tn7 was placed behind the nos promoter; transformed plant cells were resistant to methotrexate. Furthermore, L. Herrera-Estrella et al. (1983) Nature 303:209-213, have obtained expression in plant cells of enzymatic activity of octopine synthase and chloramphenicol acetyltransferase by placing their structural genes under control of nos promoters. G. Helmer et al. (1984) Biotechnol. 2:520-527, have created a fusion gene useful as a screenable marker having the promoter and 5'-end of the nos structural gene fused to E. coli .beta.-galactosidase (lacZ) sequences.
N. Murai et al. (1983) Science 222:476-482, reported fusion of the promoter and the 5'-end of the octopine synthase structural gene to a phaseolin structural gene. The encoded fusion protein was produced under control of the T-DNA promoter. Phaseolin-derived introns underwent proper post-transcriptional processing.